1. Field of the Invention
The present invention relates generally to a mobile communication system using Hybrid Automatic Repeat reQuest (HARQ), and in particular, to an apparatus and method for mapping modulation symbols of an HARQ subpacket to resources.
2. Description of the Related Art
Recently, in mobile communication systems, intensive research is being conducted on Orthogonal Frequency Division Multiplexing (OFDM), which is suitable for high-speed data transmission in wire/wireless channels. OFDM, a scheme for transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams and modulates each of them with multiple orthogonal subcarriers, or multiple orthogonal subcarrier channels, before transmission. A system that utilizes OFDM as its basic transmission scheme and distinguishes several users through the multiple subcarriers, in other words, a system that supports several users by allocating different subcarriers to different users, is commonly called Orthogonal Frequency Division Multiple Access (OFDMA) system.
HARQ is an important technology used for increasing reliability of data transmission and data throughput in packet-based mobile communication systems. HARQ refers to a combined technology of Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC).
ARQ is a technology widely used in wire/wireless data communication systems. In this technology, a data transmitter assigns sequence numbers to transmission data packets according to a predetermined rule before transmission, and a data receiver sends, to the transmitter, a retransmission request for a packet corresponding to a missing sequence number, if any, among the received packets with sequence numbers, thereby achieving reliable data transmission.
FEC is a technology for adding redundant bits to transmission data according to a predetermined rule like convolutional coding or turbo coding, before transmission, making it possible to overcome the error generated in the noise or fading environment happening in the data transmission/reception process and thus, to demodulate the originally transmitted data.
In the system using the combined HARQ of the two technologies ARQ and FEC, a data receiver performs a Cyclic Redundancy Check (CRC) check on the decoded data by performing a predetermined inverse FEC process on the received data, to determine if there is any error in the decoded data. If there is no error, the receiver feeds an Acknowledgement (ACK) back to the transmitter so that the transmitter transmits the next data packet. However, if there is an error in the data, the receiver feeds a Non-Acknowledgement (NACK) back to the transmitter so that the transmitter retransmits the previously transmitted packet. The receiver combines the retransmitted packet with the previously transmitted packet, thereby obtaining energy gain. As a result, HARQ obtains highly improved performance, as compared with the conventional ARQ that uses no combining process.
FIG. 1 is a diagram illustrating HARQ. In FIG. 1, the horizontal axis represents a time domain, and ‘data channel’ represents a channel over which a data packet is transmitted.
Referring to FIG. 1, as packet data undergoes initial transmission 101, a receiver, upon receiving the data, attempts demodulation on the initial transmission packet 101, and determines if there is a reception error on the data channel 101 in a demodulation process. If it is determined that the demodulation on the transmission data has not been successfully achieved, the receiver feeds a NACK 102 back to a data transmitter. The presence of an error can be determined through CRC check. Upon receipt of the NACK 102, the data transmitter performs packet data retransmission 103 for the initial transmission 101. Herein, even when the same information is transmitted, its redundancies can be different coded symbols.
Data transmissions 101, 103, and 105 for transmitting the same data packet are called herein “subpacket”. Upon receiving the first data retransmission 103, the data receiver performs combining on the first data retransmission 103 with the received initial transmission data 101 according to a predetermined rule, and attempts demodulation of a data channel through the combining result. If it is determined through CRC check on the data channel that the demodulation on the data transmission has failed, the receiver feeds a NACK 104 back to the data transmitter again.
Upon receipt of the NACK 104, the data transmitter performs second packet data retransmission 105, after a predetermined time has elapsed from the time of the first retransmission 103. That is, data channels for the initial packet transmission 101, the first packet retransmission 103, and the second packet retransmission 105 all transmit the same information.
Upon receiving data through the second retransmission 105, the receiver performs combining on the initial transmission 101, the first retransmission 103, and the second retransmission 105 according to a predetermined rule, and performs demodulation of the data channel. If it is determined through CRC check on the data channel that the demodulation on the data transmission is successful, the data receiver feeds an ACK 106 back to the data transmitter.
Upon receipt of the ACK 106, the data transmitter transmits the next data information, i.e., an initial transmission subpacket 107 for the second data packet, together with a control channel.
The “subpacket generation” (or subpacket construction) indicates a process of encoding a given data packet according to a predetermined procedure and then selecting some or all of the coded symbols to generate each subpacket. Although there are various possible subpacket generation methods, a subpacket generation method based on a circular buffer will be described herein, by way of example.
FIG. 2 is a diagram illustrating exemplary subpacket generation based on a circular buffer. Referring to FIG. 2, one code block 201 indicates one packet data that a transmitter intends to transmit at a given time. The code block 201 is input to a specific turbo encoder 202, which outputs specific coded symbols of S 203, P1 204, and P2 205. The S 203, P1 204, and P2 205 indicate systematic bits, parity bits #1, and parity bits #2, respectively.
The S, P1, and P2 undergo sub-block interleavers 206, 207, and 208, respectively, determining the finally interleaved symbols 209 and 210.
The interleaved symbols 209 and 210 are called a circular buffer, as illustrated in FIG. 2, because during an HARQ operation, generation of symbols for each subpacket is achieved by selecting consecutive symbols in the circular buffer, and when symbols to be sent in a particular subpacket are transferred to the circular buffer, the symbols are selected again at the start point of the circular buffer.
Referring to FIG. 2, reference numeral 211 indicates symbol generation for an initial transmission packet, reference numeral 212 indicates symbol generation for a first retransmission packet, and reference numeral 213 indicates symbol generation for a second retransmission packet.
Although the subpacket generation method illustrated in FIG. 2 may generate adjacent subpackets with inconsecutive symbols 211-213 of FIG. 2, by way of example, the subpacket generation method can also generate the adjacent subpackets with consecutive symbols.
FIG. 3 is a diagram illustrating a detailed example where resources are allocated for pilot, control information and data transmission in one subframe, which is used as a basic unit for a definition of a Transmission Time Interval (TTI) for data transmission in a downlink of a common OFDMA system.
In FIG. 3, the horizontal axis represents a frequency domain, and the vertical axis represents a time domain. The smallest square represents one subcarrier corresponding to one OFDM symbol, and the square is also called a “Resource element (RE)” for convenience. Although it is assumed in FIG. 3 that there are only 36 subcarriers in the frequency domain, by way of example, more subcarriers may exist in the actual system.
The lattices denoted by T1 represent REs where pilot symbols (or reference signals) for a transmit antenna #1 are transmitted. The lattices denoted by T2 represent REs where pilot symbols for a transmit antenna #2 are transmitted. The lattices denoted by T3 represent REs where pilot symbols for a transmit antenna #3 are transmitted. The lattices denoted by T4 represent REs where pilot symbols for a transmit antenna #4 are transmitted.
Resource blocks 302 are each a unit for resource allocation, and one resource block includes 12 subcarriers in the frequency domain and 14 OFDM symbols 301 in the time domain.
Because FIG. 3 includes a total of 36 subcarriers, there are three resource blocks. In FIG. 3, the lattices included in N OFDM symbols 303 represent REs used for transmitting control information. For convenience, these lattices will be referred to herein as a “control channel region”.
In the OFDMA system, the common control information includes downlink resource allocation information, uplink resource allocation information, and uplink power control information, and the detailed control information may be slightly different for every system.
FIG. 3 illustrates a mapping diagram in a system that uses Time Division Multiplexing (TDM) in transmitting control information. As illustrated at reference numeral 303 of FIG. 3, control information is transmitted through the foremost N OFDM symbols in the resource blocks. The ‘N’ value is generally subject to change according to the amount of the control information and the number of REs used for transmission of the control information.
Although the ‘N’ value is 3 in FIG. 3, this can be changed for every subframe, and information on the ‘N’ value is transmitted through the control channel region in every subframe. After the generation of the control channel region for transmission of the control information, the three resource blocks illustrated in FIG. 3 are allocated to terminals through predetermined scheduling. For example, resources are allocated in such a manner that among the three resource blocks, a resource block #1 is allocated to a terminal #1, a resource block #2 is allocated to a terminal #2, and a resource block #3 is allocated to a terminal #3. The resource block allocation may be changed for every subframe, and the resource block allocation information, one of the above-stated control information, is transmitted through the control channel region in every subframe.
Actually, it is common that coded symbols undergo a modulation process (QPSK, 16 QAM, etc.) before being loaded on REs. In this case, although an expression “modulated data symbols are mapped to resources” rather than an expression “coded data symbols are mapped to resources” may be correct, an expression “coded symbols are mapped to (or loaded on) resources” will be used herein for convenience. However, it would be obvious to those skilled in the art that the same can be applied in the same way even when modulated data symbols are mapped to resources.
Referring to FIG. 3, numerals indicated by reference numeral 304 indicate 14 OFDM symbols in one subframe. In FIG. 3, the control channel region includes OFDM symbols #1, #2, and #3, and when a resource block #1 is allocated, data symbols are transmitted from an OFDM symbol #4 in the resource block #1.
Among the symbols corresponding to control symbols, the leftmost 12 symbols, e.g., symbols #1-#12, are loaded on 12 subcarriers corresponding to the resource block #1 in each of OFDM symbols #1-#3, and data symbols are loaded on 12 subcarriers corresponding to the resource block #1 in an OFDM symbol #4. In this case, although the loading order (or symbol mapping order) within one OFDM symbol can be defined in various ways, it will be assumed herein that the symbols are loaded on the subcarriers in regular order.
Because 8 subcarriers, not including the subcarriers used for pilot transmission, are available for data transmission in an OFDM symbol #5, the next 8 symbols, i.e., symbols #13-#20, among the symbols corresponding to the sub-block interleaver 206, are loaded on the OFDM symbol #5 in order.
Because 12 subcarriers are available for data transmission in an OFDM symbol #6, the next 12 symbols, i.e., symbols #21-#32, among the symbols corresponding to the sub-block interleaver 206, are loaded on the OFDM symbol #6 in order. Through the same process, the symbols corresponding to the sub-block interleaver 206 are carried on all REs available in the resource block #1 in order.
In the foregoing conventional technology, the method in which a base station maps the coded symbols that it should transmit to a particular user, i.e., into resource blocks allocated to the user, undergoes a significant change in every subframe according to the size of the control channel region, i.e., according to the number, indicated by the ‘N’ value, of OFDM symbols used for transmission of control channels in a corresponding subframe. Therefore, when an error occurs during reception of information on the control channel region at a terminal, demodulation on the transmitted data packet can be almost impossible.